The present invention relates to optical sensors utilized in conjunction with a vehicle service system to acquire data, and in particular, to an improved optical sensor for use with a vehicle service system, such as a vehicle wheel alignment system, vehicle wheel balancer system, or vehicle tire changing system, which incorporates a variable lens in an optical sensor system, and which is capable of altering a lens configuration to vary a lens characteristic such as a focal length, focus, depth of field, a lens aperture, or an optical pathway.
Vehicle service systems which utilize optical sensors, such as the vehicle wheel alignment systems, vehicle tire changers, and vehicle wheel balancers, generally rely upon optical sensors which incorporate fixed lenses designed to view objects or features within a predetermined field of view. Optical sensors utilizing fixed lenses generally compromise high image resolution and accuracy to accommodate the entire predetermined field of view, even though the objects or features which are of interest generally do not encompass the entire field of view. Rather, the objects or features, such as an alignment target mounted to a vehicle wheel assembly, typically only occupy a small portion of the sensor's field of view. However, since the specific location of the object or feature within the field of view can vary, the optical sensor is required to have a field of view which is substantially larger than the object or feature, enabling the object or feature to be imaged at varied locations.
In vehicle wheel alignment systems, the goal of aligning vehicle wheels to within specific tolerances is important for optimal control of the vehicle and for consistent wear of the vehicle's tires. Alignment is performed primarily by adjusting camber, caster, toe, and steering axis inclination. As part of calculating the alignment angles for the vehicle, the angles of the wheels must be determined. The angles can be determined relative to an external reference, such as found in machine-vision vehicle wheel alignment systems, or relative to the other wheels on the vehicle, such as found in wheel-mounted vehicle wheel alignment systems. It is known that alignment angles can be measured using electro-optical transducers which incorporate solid state detector arrays. In the case of machine-vision vehicle wheel alignment systems, the detector arrays may have multiple columns and rows forming an area to capture a two-dimensional image, and in the case of wheel-mounted alignment systems, the detector array may only need to be linear, having a single row with as few as two receptor elements. In either case, the images formed on the detector arrays must be analyzed meticulously so that accurate alignment angles can be calculated.
Wheel-mounted alignment systems typically provide alignment angle sensor heads on each wheel of the vehicle, with each sensor head including an emitter and a fixed lens receiver that works in combination with at least one other sensor head along the vehicle's sides and across the vehicle. The receiver units may have photodiodes as set forth in U.S. Pat. No. 4,302,104, or a charge coupled device (CCD) as set forth in U.S. Pat. Nos. 5,018,853 and 5,519,489. The emitter units may have a single illumination source as in U.S. Pat. Nos. 4,302,104 and 5,018,853, or may incorporate multiple illumination sources as shown in U.S. Pat. No. 5,488,471. Angles and distances are calculated according to the positions of projected spots or lines that are detected by the linear arrays.
Machine-vision vehicle wheel alignment systems typically use solid state imaging sensors with fixed lenses mounted away from the vehicle to obtain images of wheel-mounted alignment targets. Each alignment target may incorporate an accurately reproduced pattern that has known control features, as set forth in U.S. Pat. No. 6,064,750. The position of the features in the image are found and an orientation of the wheel is calculated there from using well known algorithms. Some machine-vision systems do not use a predefined target but identify either random or predetermined geometric features directly on the wheel or tire of a wheel assembly, such as projected light stripes or the circular wheel rim, and use the distortion of the geometry to determine positions and orientations.
With machine-vision vehicle wheel alignment systems, the sensor imaging requirements are somewhat different from those associated with acquiring images using a standard photographic camera. Very precise measurements must be made at a rate of at least 2 Hz. on static or very nearly static scenes. This requires stable, low-noise images. The accuracy of the measurement depends on the precision with which image features such as spots, edges, centroids, corners, lines or boundaries can be determined. Methods for analyzing the images obtained using a standard area imaging sensor must take into account the possible sources of inaccuracy and compensate for them.
For example, an optical sensor, such as the DSP-600 Vision Sensor, utilized in conjunction with a vehicle wheel alignment system, such as the 611 Wheel Alignment System, both of which are manufactured and sold by Hunter Engineering Co. of Bridgeton, Mo., must have a field of view which is sufficiently large enough to view alignment targets mounted to the rear wheels of vehicles having different wheelbase lengths which range from a predetermined minimum to a predetermined maximum length. Similarly, optical sensors utilized in conjunction with vehicle wheel balancing systems and vehicle tire changing systems must have fields of view which are sufficiently large enough to view wheel rim or tire surfaces for vehicle wheel assemblies having different wheel rim diameters and different tire sizes.
Accordingly, it would be advantageous to provide a vehicle service system, such as a wheel alignment system, a vehicle wheel balancing system, or a vehicle tire changer with compact variable lens imaging sensors which are capable of adjusting one or more lens assembly optical characteristics, such as a field of view or zoom, a lens assembly optical axis, an image focus. a lens assembly aperture, or a lens assembly depth of field.
Fluid-lenses, which utilize an interface between two immiscible fluids having different refractive indices as a lens to focus incoming light have recently been developed as a form of variable lens system. One of the two fluids within the fluid-lens is an electrically conducting aqueous solution, and the other is a non-electrically conductive oil. The fluids are contained within a short tube or cylinder, with transparent end caps. The internal surfaces of the tube wall and one of the end caps are coated with a hydrophobic coating which repels the aqueous solution, resulting in the formation of a hemispherical fluid mass at the opposite end of the tube. The curved transitional interface between the aqueous solution and the oil acts to focus the incoming light in substantially the same manner as a spherically curved lens.
As shown in FIGS. 1A-1C, the shape of the fluid lens may be adjusted by applying an electric field across the hydrophobic coating such that it becomes less hydrophobic, i.e., a process called “electro-wetting”, which results from an electrically induced change in the surface-tension of the fluids. As a result of the change in surface-tension, the aqueous solution begins to wet the sidewalls of the tube, altering the radius of curvature of the interface between the two fluids, and hence, the focal length of the lens. By increasing the applied electric field, the initially convex interface can be made completely flat or even concave, resulting in a fluid-lens which transitions smoothly in a controlled manner from being convergent to divergent. An exemplary fluid-lens developed by Philips Research Laboratories in Eindhoven, Netherlands is approximately 3.0 mm in diameter, 2.2 mm in length, and has a focal range from 5.0 cm to infinity.
Combining two or more fluid-lenses along a common optical axis in a stacked or barrel configuration provides the ability to provide a zoom feature similar to that found in traditional optical lens arrangements with moving lens elements. Each fluid-lens in the stack is controlled independently.
The optical axis of a fluid-lens can be altered or tilted by varying the applied voltages about the circumference of the lens, resulting in the interface between the two fluids to distort from a symmetrical convex or concave shape, as biased by the varied applied voltage.
Accordingly, it would be advantageous to provide a vehicle service system, such as a wheel alignment system, a vehicle wheel balancing system, or a vehicle tire changer with compact variable lens imaging sensors, such as fluid-lenses, or a set of repositionable solid lenses, which are capable of adjusting a field of view, adjusting an image focus, altering a depth of field, changing a lens aperture, or varying an optical axis along which an image is viewed.